Mold needs moisture, a surface and time. Western Washington supplies persistent ambient humidity for much of the year, which means the margin between a normal house and a problem house is narrower here than in most of the country.
Extraction is the first line
Bathrooms and kitchens generate large amounts of moisture in short bursts, and getting it out of the building is the single most effective control available.
Three things to confirm. That the fan is adequately sized for the room. That the duct terminates outside the building, not in an attic, soffit or void — a surprisingly common installation fault. That the run is short and direct, since long convoluted ducts move far less air than their rating suggests.
Then actually use it. Running the fan during a shower and for a period afterwards does more than any other single habit.
The crawl space
The most common underlying cause in Western Washington homes, and the least noticed because nobody goes down there.
Air moves upward through a house. A chronically damp crawl space raises humidity throughout the living space above, which shows up as musty smells, cold floors and windows fogging more than they should — long before anything visible appears.
Check for standing water, damp ground, condensation on pipes and an intact ground vapour barrier. Address the exterior water source, because fixing the space without fixing the cause is temporary.
Fix leaks quickly
Materials dried within roughly 24 to 48 hours can usually be saved. The same materials wet for several days often cannot, and the situation shifts from drying to remediation.
That window is tighter here than the general advice suggests, because high ambient humidity means materials dry more slowly. Opening windows achieves less in a Puget Sound autumn than the same advice would in a dry climate, since outside air is frequently as damp as inside air.
Watch the cold surfaces
Mold appears first where surfaces are coldest, because that is where moisture condenses. Exterior wall corners, behind furniture pushed against outside walls, in unheated rooms, around single-glazed windows and in closets on exterior walls.
Improving insulation raises surface temperatures, which is why insulation is a mold control as well as a heating one. Leaving a gap behind furniture on exterior walls is a free version of the same idea.
Windows and condensation
Condensation on the inside of glass usually means indoor humidity is high, not that the window is defective — the glass is simply the coldest surface. Persistent heavy condensation is a signal worth acting on.
Moisture between the panes is different: that is a failed seal, and it means the unit needs replacing.
Drying laundry indoors
A genuine contributor that gets overlooked. Drying laundry inside without ventilation puts a large amount of moisture into the house, which in a marine climate has nowhere convenient to go.
Confirm the tumble dryer vents outside if it is a vented type, and that the duct is clear.
When it is more than prevention
Visible growth over a substantial area, growth that returns after cleaning, or any growth accompanied by a persistent musty smell you cannot locate all warrant a professional assessment.
Growth returning after cleaning is the important signal: it means the moisture source is still active, and cleaning the surface without removing the cause achieves nothing.